In a magnetic tape cartridge, for example of the type described in U.S. Pat. No. 4,571,789, in which the tape moves bidirectionally at high speeds between two reels, it is important to avoid physical distortion of the cartridge which can be caused by differential thermal expansion between the plastic cover and aluminum base plate. Otherwise, accurate tracking of the tape by a transducer head inserted into the cartridge is degraded.
Such tape cartridges typically comprise a thin aluminum base plate and a polymeric cover. The tape reels and tape guides are generally mounted on the base plate.
The aforementioned U.S. Pat. No. 4,571,789 describes a snap fastener which connects the cartridge cover and base plate while allowing the cover and plate to expand at different rates without greatly deforming the cartridge. More particularly, the snap fastener is described as including a solid body portion attached to the cover, a bifurcation defined by a slot, and two spring arms extending from the bifurcation toward a hole in the base plate, the spring arms being capable of resilient deflection toward each other.
The arms terminate in enlarged heads for insertion through the base plate hole, which heads have a dimension parallel to the slot equal to or less than the hole, and an undeflected dimension transverse to the slot greater than the hole. The patent describes that insertion of the enlarged heads into the hole causes deflection of the spring arms toward each other allowing passage of the heads through the hole and permitting the resilient return of the spring arms to the undeflected position wherby the snap fastener is retained in the hole.
Since these snap fasteners effect a somewhat flexible connection between the cover and the base plate, they reportedly reduce the deformation or bowing of the cover and the base plate, due to forces induced by differential thermal expansion, as compared to a rigid connection such as a screw fastener. The somewhat flexible connection is attributable to the bifurcation of the snap fastener which allows the fastener to bend. The same fastener bifurcation, however, also has to secure the cover to the base plate. These requirements are somewhat contradictory because for minimum thermal distortion the fastener should be as flexible as possible but for maximum hold down force, it should be as stiff as possible.